The present invention relates to a device incorporated in image forming equipment for transporting a paper sheet and, more particularly, to a paper transporting device having a plurality of paper feeding sections arranged one above and a common vertically extending transport path for transporting a paper sheet fed from any one of the paper feeding sections vertically therealong.
Many of modern image forming equipment such as copiers, facsimile transceivers and printers have a plurality of paper feeding sections each being loaded with paper sheets of particular size. In this type of equipment, a paper sheet fed from any one of the paper feeding sections is transported along a predetermined transport path to an image transfer station, so that a toner image formed on a photoconductive element may be transferred to the paper sheet. The space available in such equipment is becoming smaller and smaller due to the increasing demand for a miniature configuration. In light of this, Japanese Utility Model Laid-Open Publication No. 78629/1989, for example, discloses a paper transport arrangement wherein a single common transport path extends vertically upward to feed all the paper sheets fed from individual paper feeding sections to an image transfer station therealong. This arrangement has a plurality of transport rollers made of rubber and positioned along the vertical transport path at intervals which are smaller than the minimum paper size as measured in the intended direction of transport. Guide members each guides a paper sheet between nearby ones of the transport rollers. The transport rollers are driven by an exclusive drive mechanism. A drawback with this conventional arrangement is, however, that the coefficient of friction of the transport rollers made of rubber changes due to aging and thereby causes paper sheets to slip on the rollers. To eliminate such slippage, a pressure of 1 kgf to 3 kgf is constantly applied to the individual transport rollers. When the length of the transport path and, therefore, the number of transport rollers increases, such pressures act on the drive mechanism as heavy loads. Another drawback is that when a paper sheet is curled due to humid environment, it is often caught by the gap between the transport roller and guide plate which neighbor each other, jamming the transport path.
Generally, the transport of a paper sheet using a transport roller relies on the coefficient of friction of the roller surface, the pressure which the roller exerts on a paper sheet, the surface hardness of the roller which is made of rubber, etc. Since the area over which the roller and paper sheet can contact each other is limited, the paper sheet is apt to slip on the roller when a load acts on the paper sheet in the opposite direction to the transport direction. To eliminate this problem, it has been customary to locate a register roller in front of a paper sheet and to cause a paper sheet to abut against a rubber roller which is biased by a substantial pressure. Specifically, a paper sheet is caused to warp on abutting against such a rubber roller to correct a skew and is then driven by the register roller at a particular timing matching the operation of an image transfer device. This kind of implementation may achieve a satisfactory paper transport ability in the initial stage of use. However, the surface of the rubber roller sequentially wears to reduce the coefficient of friction as well as the diameter thereof. This, coupled with the fact that paper dust deposited on the roller surface reduces the contact area of the roller surface and paper sheet, critically lowers the paper transport ability.